Not denying that the process happens, and maybe it's irreconcilable. But I'll attempt to describe my position in more general terms.
The difference between "literal" and "figurative"/"metaphorical" is whether a term is being used as a first-order label for a thing in itself, or as a reference to another concept. That is, a figurative use of a term applies meaning by drawing on a distinct source of meaning.
Words (or idiomatic phrases) can become literal when their usage becomes so strongly identified with what they label that their meaning is no longer determined by reference. "Turn on" no longer involves turning, for example.
I agree that "definition" has become literal, because we no longer refer to the concept of boundaries each time we encounter the term. So too with "cradle": we imagine a particular object that isn't a basket, in itself.
I do not believe this is the case when someone calls the Yellow River the cradle of Chinese civilization. I believe that people hearing that term primarily understand "cradle" by reference to the baby-related object, rather than because the word has meaning in itself relating to the start of civilization. As evidence of this, I think attaching "of Chinese civilization" is doing meaningful work, and that most people would be thrown off by the abstract phrase "cradles around the world" or even by the more specific phrase "the Yellow River, China's cradle." They may still understand the speaker's meaning, but they would do so by connecting the metaphorical dots, not because "cradle" has become a literalism.